Sirius looks more like his mother than his father, and Regulus is just the opposite (yes, Orion and Walburga shared a shit ton of relatives, but I think they looked quite different)
all the siblings rushed to Andromeda's room whenever they were scared. Be it baby Sirius when the thunder was so loud he couldn't stop trembling, or Narcissa when she was awaiting her OWL results, or Regulus when he felt like he had lost Sirius forever.
Bellatrix was the one who got crucio'd the most. Yes, she was obedient to Voldemort, yes, she supported the dark arts, but her obedience came with a price that was her sanity.
the marauders weren't as sneaky as they thought they were during their first year. It was Narcissa who fine tuned their pranks (she did not like that Sirius was hanging out with blood-traitors, but she was a prankster too) and ensured that they did not get caught.
Regulus sleeps on his arms. He tries to sleep on his back, but for some reason, it never works out.
the reason for the above is that when Sirius and Regulus shared a room (as toddlers), Sirius would always pull Regulus closer to him when the latter was asleep. Many times, they would face each other, Regulus' arms squished between his and Sirius' body and Sirius' arms around his. Regulus does not know this.
Sirius makes time to stargaze at least once a week. When it first started, it was a means of spotting the constellations/stars of all the members in his family, and soon, it became a means of escape the Black brothers used very often.
Sirius also recites names of stars and constellations to keep himself from losing his mind whenever he is crucio'd. This was also what kept him alive in Azkaban apart from redemption.
Narcissa always dutch braids her hair. It annoyed her parents, because they are of French ethnicity, but it was one of the many styles that made her sharp features stand out.
after running away with Ted, Andromeda couldn't sleep for weeks because she fully expected that one of her family members would somehow find her and kill her, or worse, drag her back.
a similar thing took place right after Dora was born for obvious reasons.
Bellatrix loved to play the electric guitar. Her parents didn't know about the guitar she had hid under the floorboards of her room and certainly did not know about the complex magic she had used to start the guitar without the use of electricity (even better, she developed it when she was underage)
Orion's hair and beard were always groomed. Everyone assumed it was magic, but Regulus was the only one who knew that it was due to Sleakzey's, Fleamont Potter's creation. He had been threatened with having to see Sirius getting tortured for three full hours, which was the only reason he carried this secret to the grave.
on a similar note, SIrius also saw Walburga using Sleakzey's to tame her hair. Walburga, however, did not know he knew.
the Blacks always dined together. Even if someone was sent to their room or grounded, they made it a point to ensure everyone important was there to eat their meals. These meals were also frequented by the portraits, and a full extended family conversation would nearly always take place.
the Blacks also had abysmal immunity (incest and good immunity do not go hand in hand in this case). However, Kreacher was great at healing charms, so it was very tough to tell when someone was actually sick.
every single Black sibling was not straight, apart from Andromeda, who would throw her fists at anyone who showed even a smidge of homophobia.
Walburga had developed a counter-charm for the permanent fixing charm, and when Sirius found out, he stayed up for five whole days to successfully find a way to make something impervious to Walburga's counter charm.
However, he had forgotten about it during his time in Azkaban, and didn't have the energy to recover it after that.
Andromeda's favourite colour was green. Not the green that characterised slytherin (she thought it was elegant, but it wasn't her taste), but the light green that symbolises new life.
Bellatrix tried to run away once. It did not work.
Regulus, though he loves rings, hates that they sometimes get stained by ink. Therefore, he started wearing them as necklaces, and it isn't rare to see him loosening them during the day.
he did not risk it at home though, especially after he was declared the heir.